1. Field of the Invention
The present invention relates to a fixing unit for use in an image forming apparatus such as a copying machine, a printer and a facsimile applying an electrophotographic or electrostatic recording scheme and to an image forming apparatus using the same. More specifically, present invention relates to a fixing unit capable of always performing a good fixing process regardless of the type of a transfer medium and an image forming mode and to an image forming apparatus using the same.
2. Related Art Statement
Hitherto, there has been a unit as shown in FIG. 17 for example as a fixing unit for heating and fixing a non-fixed toner image which has been transferred to a copy sheet in an image forming apparatus such as a copying machine, a printer and a facsimile applying electrophotographic or electrostatic recording scheme. As shown in FIG. 17, the fixing unit is constructed so as to melt and fix a toner image 103 on a copy sheet 104 by heating and pressing it by passing the copy sheet 104 carrying the non-fixed toner image 103 through a pressure-contact area between a pair of rollers 101 and 102 wherein at least one roller has a heating source.
In FIG. 17, the reference numeral (101) denotes a fixing roller and (102) a pressure roller. The fixing roller 101 is structured by coating a relatively thick heat-resistant elastic layer 112 made of silicon rubber or the like and a top coat layer 113 made of fluorine rubber or the like on the surface of a metal hollow core 111 made of aluminum or the like whose thermal conductivity is high. A halogen lamp 114 is disposed as a heating source within the metal hollow core 111 and is turned on/off by a temperature control circuit not shown based on a signal from a temperature sensor 115 provided on the surface of the fixing roller 101 to control the surface at predetermined temperature. Further, an oil supply unit 105 for supplying a certain amount of silicon oil is provided on the surface of the fixing roller 101 to prevent a part of the non-fixed toner image 103 on the copy sheet 104 from being transferred to the fixing roller 101 (hereinafter referred to as xe2x80x9coffsetxe2x80x9d) during fixing. A cleaning unit 106 for removing offset toner and the like is also provided on the surface of the fixing roller 101.
Meanwhile, the pressure roller 102 is structured by coating a heat-resistant elastic layer 122 which is made of silicon rubber or the like and which is relatively thinner than the elastic layer 112 of the fixing roller 101 and a top coat layer 123 made of fluorine rubber or the like on the surface of a metal hollow core 121 made of aluminum or the like whose thermal conductivity is high. A halogen lamp 124 is disposed as a heating source within the metal hollow core 121 and is turned on/off by the temperature control circuit not shown based on a signal from a temperature sensor 125 provided on the surface of the pressure roller 102 to control the surface at predetermined temperature.
In the heating roller type fixing unit constructed as described above, a pressure-contact part (hereinafter referred to as a xe2x80x9cnipxe2x80x9d) 107 is created by elastic deformation of the elastic layer 112 of the fixing roller 101. The non-fixed toner image 103 is melted and is fixed on the copy sheet 104 by thermal energy and pressure as the copy sheet 104 carrying the non-fixed toner image 103 passes through the nip 107 in such fixing unit.
However, the prior art technology described above has had the following problems. While the fixing roller 101 and the pressure roller 102 have the elastic layers 112 and 122 in the fixing unit constructed as described above, these elastic layers 112 and 122 are made of elastic member such as silicon rubber which has a certain thickness and whose thermal conductivity is relatively low. Therefore, when the surface temperature of the fixing roller 101 or of the pressure roller 102 drops below the predetermined temperature while feeding papers, there has been a possibility in the fixing unit described above that it takes time to transmit heat of the halogen lamps 114 and 124 to the surface via the elastic layers 112 and 122, thus causing fixing failure, even if the temperature sensors 115 and 125 detect the drop of the temperature and power is fed to the halogen lamps 114 and 124. The fixing failure is liable to occur when processing speed increases in particular and there has been a problem that continuous printing is limited in order to prevent the fixing failure from occurring.
Then, in order to solve such problems, there has been already proposed a technology for slowing down the drop of the temperature of the surface of the fixing roller by abutting an external heating roller whose temperature is kept high to the surface of the fixing roller as disclosed in Japanese Patent Laid-Open Nos. Hei. 10-149044 and Hei. 11-24489.
However, in case of the technology disclosed in Japanese Patent Laid-Open Nos. Hei. 10-149044 and Hei. 11-24489 described above, the external heating roller whose temperature is kept high is abutted to the surface of the fixing roller immediately when the sensor detects that the surface temperature of the fixing roller has dropped even though the optimum surface temperature of the fixing roller is different depending on the type of the copy sheet 104 and on image forming modes such as black-and-white mode and color mode. Therefore, there has been a case when the temperature of the fixing roller in fixing papers differs considerably from temperature suitable as conditions for fixing papers such as the copy sheet depending on the temperature of the external heating roller and the fixing roller. Accordingly, the technology disclosed in Japanese Patent Laid-Open Nos. Hei. 10-149044 and Hei. 11-24489 has had a possibility that it may cause new problems such as an increase of offset toner, a failure in peeling OHP sheets and insufficient fixing.
More specifically, in case of the technology disclosed in Japanese Patent Laid-Open Nos. Hei. 10-149044 and Hei. 11-24489, the surface temperature of the fixing roller 101 rises considerably above the predetermined temperature when the external heating roller not shown whose surface temperature is high abuts to the surface of the fixing roller 101 in the process when the surface temperature of the fixing roller 101 rises due to temporal delay of the increase of the surface temperature of the fixing roller 101 as shown in FIG. 18. Then, because the surface temperature of the fixing roller 101 is considerably higher than the predetermined temperature, it has caused problems that the non-fixed toner image 103 to be fixed on the copy sheet 104 melts excessively, thus dropping the gross, an amount of toner transferred to the surface of the fixing roller 101 increases (called as hot offset), and the deterioration of the elastic layer 112 and the top coat layer 113 of the fixing roller 101 is accelerated. Still more, when the surface temperature of the fixing roller 101 is considerably higher than the predetermined temperature and when a transfer medium 104 made of a synthetic resin film called as an OHP sheet and a tack film is used, there has been a problem that those synthetic resin films such as the OHP sheet and the tack film are softened and are wrapped around the surface of the fixing roller 101, thus making it impossible to perform the fixing process. Meanwhile, there has been a case when the surface temperature of the fixing roller 101 is lower than the predetermined temperature, thus causing insufficient fixing, depending on the surface temperature of the fixing roller 101 and the surface temperature of the external heating roller.
As a second problem, there has been a case when the surface temperature of the fixing roller 101 and the pressure roller 102 drops gradually as the heat of the fixing roller 101 and the pressure roller 102 is taken away gradually by the copy sheets 104 when a continuous run job of a large number of sheets is executed by performing the fixing process of the large number of copy sheets continuously by the fixing unit as shown in FIG. 17. A type of machine in which power consumption of the fixing unit is required to be low shows this tendency remarkably because the halogen lamps whose power consumption is large cannot be used as the heating sources 114 and 124. Accordingly, when the next continuous run job of a large number of sheets is carried out under the condition in which the temperature of the fixing roller 101 and the pressure roller 102 has dropped due to the previous continuous run job, the temperature of the rollers drops below the lowest fixing temperature during the job and the temperature must be recovered by stopping the run.
However, the elastic layers 112 and 122 of the fixing roller 101 and the pressure roller 102 for color fixing have had the problems as described before that because the thermal conductivity is relatively low, the recovery of temperature of the surface of the roller after feeding the sheets is slow and the continuous job cannot be executed. Still more, it has had a problems that the surface temperature of the fixing roller 101 and the pressure roller 102 rises considerably above the set temperature this time after recovering the surface temperature of the fixing roller 101 and the pressure roller 102 to the set temperature and an overshoot is large, thus causing the increase of toner offset and the peeling defective jam of OHP sheets or the like as described before when the copy sheets 104 are fed in the state when the temperature has risen excessively.
Then, in order to solve the above-mentioned problems, the applicant of the present invention has already proposed a technology for recovering the temperature of the fixing roller and others by utilizing the phenomenon of overshoot while preventing defective image which is otherwise caused by the overshoot as disclosed in Japanese Patent Laid-Open No. Hei. 6-11997.
However, the technology disclosed in Japanese Patent Laid-Open No. Hei. 6-11997 has had a problem that it takes a certain time for the surface temperature of the fixing roller to recover to the predetermined temperature, thus delaying the next fixing process, because it recovers the temperature of the fixing roller while utilizing the overshoot phenomenon by setting two control temperatures of first and second control temperatures as the surface temperature of the fixing roller and by switching the first and second control temperatures at predetermined timing.
The present invention has been made in order to solve the problems of the above-mentioned prior art and provides a fixing unit, and an image forming apparatus using the same, which is capable of attaining always good fixing performance by preventing the surface temperature of the fixing roller from becoming too high or too low even when it is constructed so as to heat the surface of the fixing roller by an external heating member by abutting it to the surface of the fixing roller.
The invention also provides a fixing unit, and an image forming apparatus using the same, which is capable of returning the surface temperature of the fixing roller to set temperature in a short time and of preventing an overshoot or the like from occurring even when the fixing process is continuously run.
According to a first aspect of the invention, an image forming apparatus has a fixing unit that fixes a black-and-white or color toner image which has been formed on a transfer medium by an image forming unit on the transfer medium and a control unit. The fixing unit includes a fixing roller having a heating source therein; a pressurizing member which press-contacts the fixing roller; an external heating member which heats the surface of the fixing roller from the outside; and an abutting-separating device that makes the external heating member abut or separate from the surface of the fixing roller. The control unit controls timing for making the external heating member abut the surface of the fixing roller according to at least one of the surface temperature of the external heating member and/or fixing roller, the type of the transfer medium which undergoes the fixing process by the fixing unit and an image forming mode for the transfer medium.
According to a second aspect of the invention, an image forming apparatus has a fixing unit that fixes a black-and-white or color toner image which has been formed on a transfer medium by an image forming unit on the transfer medium and a control unit. The fixing unit includes a fixing roller having a heating source therein; a pressurizing member which has a heating source therein and press-contacts the fixing roller; an external heating member which heats the surface of the fixing roller from the outside; a first abutting-separating device which makes the pressurizing member press-contact or separate from the surface of the fixing roller; and a second abutting-separating device which makes the external heating member abut or separate from the surface of the fixing roller. The control unit carries out at least one of a control of continuous abutting of the external heating member to the fixing roller when at least one of surface temperatures of the fixing roller and the pressurizing member is lower than a preset temperature and a control of continuous abutting of the pressurizing member to the fixing roller at the point of time when the fixing process by the fixing unit ends.
According to a third aspect of the invention, an external heating method is provided for heating the surface of a fixing roller having a heating source therein by making an external heating member abut the surface of the fixing roller by an abutting-separating device for making the external heating member abut or separate from the surface of the fixing roller. The method includes a temperature measuring step for measuring the surface temperature of the fixing roller; and a timing deciding step of finding the difference between the surface temperature of the fixing roller measured in the previous step and predetermined temperature and of deciding timing for making the external heating member abut the fixing roller by the abutting-separating device according to an image forming mode to make the external heating member abut the fixing roller by the abutting-separating device.
According to a fourth aspect of the invention, an external heating method using the image forming apparatus of the second aspect, the method includes the steps of measuring and inputting the surface temperatures of the fixing roller and the pressurizing member at the point of time when the fixing process by the fixing unit ends, and providing a control for making at least one of the external heating member and the pressurizing member abut or press-contact the fixing roller if at least one of surface temperatures of the fixing roller and the pressurizing member is lower than a preset temperature.